Integrated optical waveguide circuits (IOWCs) are photonics counterparts of electronic integrated circuits (ICs). They are used to perform different operations for processing optical signals in optical or opto-electronics domain. They have a broad range of applications such as in optical telecommunication networks, sensors, and biomedical apparatuses.
An IOWC has a number of input and output waveguides that have to be attached to optical fibers before it can be used by the consumers. However, the attachment of fibers to waveguides with the objective of maximizing optical power coupling between the fiber and waveguide imposes several technological challenges. That is because firstly the core diameter of fibers and waveguides are typically small, e.g. 2 to 20 microns, and secondly they are not often equal. The former makes the fiber to waveguide alignment extremely critical and the latter causes the loss of some optical power due to mismatch between the mode sizes of the fiber and waveguide. Any small misalignment between the fiber and waveguide core axis can increase the coupling loss of optical power significantly, whereas the mismatch between the core size of the fiber and waveguide will result in loss of optical power even at the perfect alignment position. However waveguides with small core diameter and higher numerical aperture than single mode fibers are more desirable in order to decrease the total length of an IOWCs. Therefore one should often compromise between the inherent joint loss of waveguides of different core sizes and the total length of the IOWCs.
There are two conventional methods of attaching fiber to waveguides, one is by manual or active alignment, and the other by automatic alignment. A combination of automatic and manual alignment is also possible. In the manual method, each individual optical fiber is abutted to the input or output waveguides by careful adjustment of the fiber position in front of the waveguide while monitoring the efficiency of the light coupling and affixing the fiber using adhesive and the like at the optimum position. Although manual alignment results in a better coupling loss, it is very time consuming thus making the packing of IOWCs very expensive. Also because of the use of adhesives used in affixing the fiber to the waveguide, the manual method does not provide very reliable results.
As for the automatic alignment method, it is known to use a substrate having alignment grooves to align the fiber core and waveguide and also to support the fiber. Alignment grooves for this purpose are also known in integrated optical circuits. While grooves provide lateral and angular alignment they cannot provide precise vertical alignment since, within a micron, precision control of the depth of the grooves is very difficult. Automatic alignment methods have been disclosed, for instance, in U.S. Pat. Nos. 4,796,975, 5,393,371, 5,175,781, 5,297,228, 5,361,382, and 5,600,745.
In U.S. Pat. No. 5,175,781 issued on Dec. 29, 1992 to Hockaday et al, there is disclosed a method of attaching fiber to integrated optic chips (IOC) by forming alignment grooves, using laser ablation on the IOC followed by cutting the surface ends of the waveguides by dicing saw, disposing optical fibers within the grooves, and securing the fibers with adhesives to attach the fiber to the IOC permanently. This method again cannot insure vertical alignment and doesn't solve the core size mismatch problem.
In U.S. Pat. No. 5,361,382 to Nakamatira et al, another method of connecting optical waveguide and optical fiber has been disclosed. In that disclosure waveguides are made on one substrate. An optical connector is formed on a second substrate. The substrates have means for aligning them together and the second substrate has alignment grooves with fibers embedded therein. Adhesive and the like are used to fix the optical connector substrate and waveguide substrate after aligning the substrates. In a subsequent U.S. Pat. No. 5,297,228 from the same applicants, fiber aligning jigs are aligned with the optical waveguides using markers and are abutted to each other by laying a common guide pin along the corresponding prefabricated pin grooves on the fiber aligning jig and waveguide substrate. This method also suffers from the low reliability of using adhesive and also the loss associated with the fiber and waveguide core mismatch.
In U.S. Pat. No. 5,600,745, issued on February 1997 to Hsinchu et al, there is disclosed a preferred method of chemically etching of V-grooves and V-grooves with a predetermined lateral angle to precisely adjust the fibers to the waveguide in the vertical direction. This method however uses adhesives to abut the fibers to the waveguide and it doesn't address the core size mismatching problem.